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Tiny terrorists: Bugs using chemical warfare and taking hostages

Body size and a potential for biological impact have little in common. Mosquitoes are far more dangerous than tigers, and some bacteria are a thousand-fold more likely to be lethal than a rattlesnake.

Small organisms can also send us important messages. The first is that the associations of even the lowest forms of life are complex and intricate. Thus, we should consider and deliberate before tampering with natural systems we do not understand. A second message is that today's scientists can do a remarkable job at unraveling the mysteries of nature, which helps us appreciate the world's biological marvels.

A chemical association involving two species of tiny marine invertebrates and a fish was discovered in Antarctica a decade ago by J.B. McClintock of the University of Alabama in Birmingham and J. Janssen of Loyola University in Chicago. One species is an amphipod, a type of invertebrate related to sow bugs, or roly-polies. Amphipods are defenseless against a small fish that is major predator on small animals in the region. In the clear waters of the Antarctic sea, the amphipod is easy prey. In contrast, another small invertebrate known as pteropod is chemically noxious to the fish and is not a prey item. Pteropods are in essence tiny, bright orange snails without shells that move through the water by flapping wing-like appendages. Field studies based on dissections of fish have shown that amphipods are common prey for a variety of fish. But pteropods are not eaten, presumably because they are distasteful. Fish that grab a pteropod will shake their heads violently and spit the animal out.

Fish avoid brightly colored, bad-tasting pteropods but readily eat amphipods. So what's an amphipod to do? Any tasty animal with no defense whatsoever would be short-lived on the evolutionary scale. Thus amphipods have developed a defense mechanism -- as long as pteropods are around. After drilling holes in the sea ice, the investigators observed many amphipods carrying pteropods on their backs. Amphipods were observed to pursue small pteropods then grasp them with pincers on their rear appendages and actually hold the smaller animal captive. The amphipod places the captured pteropod on its own back in full view, still holding it with the pincers, and goes about its business.

Presumably, amphipods capture pteropods as a chemical weapon against fish predation. The scientists ran laboratory tests to find out for sure. The experiments revealed that amphipods carrying a pteropod were immune to fish attack. Those without pteropods were readily consumed by fish. Many animals use chemical defense against predators. Skunks, scorpions, and stingrays are but a few species that produce their own chemical warfare. The amphipod-pteropod relationship is a rare example of how a prey species actively exploits another for chemical protection.

The amphipod pays a price for carrying around its kidnapped bundle of protection. With the added weight, swimming around the Arctic sea is a bit more difficult. But the benefit of not having to worry about every fish that swims by is apparently worth it. What about the pteropods? What do they get out of this relationship? Maybe nothing, or maybe we do not yet know. No pteropods were observed to die while being held hostage for periods of time up to a week, but they presumably do not maintain a normal diet. Being hauled around on another animal's back for a week or more would not seem to be in a pteropod's best interest. Ironically, the trait that spares them from fish predation is the very trait that results in their being abducted by another animal.

This complex relationship among species is fascinating, particularly because it occurs in the polar regions where species diversity is low. In regions where many species have opportunities to interact, more and more complex relationship develop. The temperate zones and tropics are teeming with bizarre and spectacular plant and animal associations that are waiting to be discovered by those with patience and imagination.

Whit Gibbons is a senior research scientist with the University of Georgia's Savannah River Ecology Lab. E-mail: gibbons@srel.edu.